1. Field of the Invention
This invention relates to water purification, and more particularly concerns apparatus and compositions for the selective removal of trace levels of dissolved metals from rapidly flowing water.
2. Description of the Prior Art
Undesirable pollutant species such as dissolved toxic metals can occur regularly or periodically in natural bodies of water such as lakes, rivers, streams, groundwater and stormwater, and in industrial waters such as landfill leachate, municipal sewage systems and wastewater discharges. The toxic metals are primarily multivalent “heavy” metals, and can be caused to form insoluble compounds with anionic additives. The most prevalent technique for the removal of toxic heavy metals is to add to the water a sufficient amount of an anionic precipitating agent. The resultant precipitate is then allowed to settle or is removed by filtration or other methods.
Although precipitative techniques are effective, they require that the water undergoing remediation be accumulated in tanks and held there for considerable periods of time with agitation while the treatment agents are added and dispersed, causing the heavy metals to form precipitates of sufficiently large particle size to facilitate settling and/or filtration. Operations of this nature require large, expensive equipment installation and high operating expense. The high cost of such installations is justifiable only where extremely large volumes of water must be treated on a long term basis, such as in municipal waste water treatment facilities and operations involving large industrial facilities.
Another general technique for removing dissolved metal species from water involves the passage of a stream of the water undergoing remediation through a stationary bed of absorbent material. Suitable absorbent materials for such on-the-run treatments include ion exchange resins having affinity for species in either cationic or anionic form. The advantage of such technique is that the water can be treated while flowing through a confined bed of the resins, thereby avoiding the need for large holding and processing tanks. However, such beds impose an impedance to the flow of water therethrough, necessitating pumping pressures of 10 to 30 psi to achieve reasonable flow, and are adversely affected by accumulated suspended matter derived from the inflow water.
Said absorbent materials must have the ability to selectively remove the heavy metals at “trace” levels of 1-10 parts per million (ppm) amidst vastly greater concentrations of commonly abundant innocuous species. An exemplary absorbent having adequate selective affinity for trace levels of heavy metal ions is hydrogel granular material having 30% to 70% by weight of water, as disclosed in U.S. Pat. Nos. 3,715,339 and 7,041,222. However, such hydrogel granules have irregular shapes, causing beds of the granules to produce very high impedance to the flow of water. In an effort to overcome the high impedance to flow, said hydrogel polymers have been deposited upon pieces of porous sponge, as disclosed in U.S. Pat. No. 5,064,540. However, when the sponge pieces are employed as a stationary bed, a long bed length is needed for efficient removal of dissolved ionic species. Another shortcoming of the sponge pieces is that they cannot be blended with granular absorbent materials.
The most prevalent need for water treatments at fast flow rates is in stormwater catch basins which receive a rapid, gravity induced flow of water, and treat the water with minimal flow impedance within a very confined space.
It is accordingly an object of the present invention to provide means for selectively removing trace levels of dissolved metals from fast flowing water.
It is another object of this invention to provide means as in the foregoing object which occupies relatively little space.
It is a further object of the present invention to provide apparatus which will accommodate granular absorbent materials.
It is yet another object of this invention to provide apparatus of the aforesaid nature which is relatively unaffected by suspended matter within the water undergoing treatment.
It is a still further object of the present invention to provide apparatus of the aforesaid nature which is inexpensive to produce and maintain.
These objects and other objects and advantages of the invention will be apparent from the following description.